A conductive polymer has such a high electric conductivity that it has been used as solid electrolyte of a solid electrolyte capacitor such as aluminum solid electrolyte capacitor, tantalum solid electrolyte capacitor and niobium solid electrolyte capacitor.
The conductive polymer in such an application can be often made from a thiophene or its derivative by means of the oxidation polymerization (i.e., chemical oxidation polymerization) because of providing high utility products having balanced their electric conductivity and heat resistance. See Patent Publications Nos. 1 and 2.
In carrying out the oxidation polymerization of the thiophene or its derivative, a dopant of an organic sulfonic acid is used. The oxidant can be a transition metal, and in particular, a ferric salt compound is said to be favorable. Usually, a ferric salt of an organic sulfonic acid can be used as an oxidant and dopant agent to carry out a chemical oxidation polymerization of a thiophene or its derivative.
In the production of solid electrolyte capacitors using the conductive polymers as solid electrolyte, for example, a capacitor element is immersed in a monomer solution and taken out therefrom, and then, the capacitor element is immersed in an oxidant and dopant solution and taken out therefrom to polymerize. Alternatively, a capacitor element is immersed in an oxidant and dopant solution and taken out therefrom, and then, the capacitor element is immersed in a monomer solution, and taken out therefrom to polymerize. Alternatively, an oxidant and dopant solution is mixed with a monomer solution, into which a capacitor element is immersed, and then, taken out therefrom to polymerize.
In the production of the solid electrolyte capacitor, however, when p-tolueneferric sulfonate, as it used to be, is added as an oxidant and dopant agent to carry out the oxidation polymerization of the thiophene or its derivative, even though solid electrolyte capacitors can be produced with a low ESR (equivalent series resistance) and a high capacitance, they have lowered the voltage resistance (breakdown voltage). Therefore, it is reported that such an oxidant and dopant agent is inappropriate in the production of solid electrolyte capacitors in high voltage application. See patent Publication No. 3.